half animal has a mirror. Can I move the tail from one side to the other?
Another alternative is to keep the mesh symmetric with the tail in the center, and then add an Armature with bones to control the tail and then pose the armature to make the tail off-center.

The order of the modifiers (mirror and armature) are probably important.

Edit:  is a .blend file illustrating the technique and proving that it is possible to pose a mirrored mesh without applying the mirror modifier (leaving it as a modifier).